1. Field of the Invention
The present invention generally relates to the recording of radar (radio detection and ranging) signal information and, more particularly, to recording of such signal information by recorder machines employing magnetic recording media.
2. State of the Art
In many situations, it is desirable to have records of information that is transmitted and received by radar systems. For example, in aircraft traffic-control systems using radar, accurate records of radar transmissions and receptions can be employed to improve the effectiveness of the systems and can be used to determine causes of mishaps. Also, it is sometimes desirable to have unmanned radar installations; for such installations, records of radar transmissions and receptions are mandatory.
Accurate recording of radar information, however, is complex. In part, such complexities arise because radar information is inherently temporal, which is to say that any recording of radar information should be reproducible in a manner that preserves original timing relationships. Thus, for example, knowledge from a record that radar detected two objects at given locations is of limited value unless a precise relationship can be established between the time of detection of the first object and the time of detection of the second object. Additional complexities arise when radar antennae are in motion, either linearly or rotationally.
In radar systems, it is typical to display radar echo signals in real time on the phosphorescent screens of pulse position indicator (PPI) machines. Synchronization for purposes of display of echo signals on PPI screens is accomplished through use of signals known as master trigger signals. Generally speaking, master trigger signals are pulses having fast-rising leading edges that are used to control PPI machines and which serve as benchmarks for detecting the periods between echo pulse transmissions and receptions. That is, master trigger signals serve as benchmarks for determining the locations of detected objects relative to transmitting radar systems. Master trigger pulses normally are of very short duration, usually about four to twelve microseconds. The number of master trigger pulses transmitted per second defines the radar pulse repetition frequency. (The time from the beginning of one master trigger pulse to the beginning of the next master trigger pulse is the pulse repetition time and is the reciprocal of the pulse repetition frequency.) Pulse repetition time is variable depending upon the selected range of the radar system, and is generally a few hundred microseconds or less.
Machines of various types have been used to record radar transmission and reception information of the type that is normally displayed on PPI machines. However, such recordings of PPI-type radar information has been problematical. One difficulty relates to resolution, i.e., the ability to record and reproduce sharp changes in reflected radar signals. If resolution is diminished upon recording or reproduction of radar information, it may be difficult or impossible during replay to discern whether a radar reflection represents a single object or multiple objects that are close together, especially if the objects are slow-moving. To provide high resolution, the leading edges of radar information signals must be clearly reproduced. Because the leading edges of such signals have very fast rise times, recording of the signals requires wide bandwidths approaching ten megahertz; by way of comparison, ordinary television signals have a bandwidth less than about six megahertz, and conventional videotape recording (VTR) machines normally record over a bandwidth of about four or five megahertz.
To obtain recordings over bandwidths appropriate for PPI-type radar information, one conventional approach has been to use VTR machines of the helical scan type that have rotary record and reproduce heads to record information in analog form at an angle to the tape transport direction. For present purposes, such machines will be called analog rotary-type VTR machines. The advantage of analog rotary-type VTR machines is that wideband radar signals can be recorded for periods of up to an hour or so on a single reel of magnetic recording tape. As configured for recording PPI-type radar signal information, such machines often have two recording channels, one of which is used for echo signal information and the other of which is used for multiplexed master trigger signal information and azimuth information. In such a configuration, the machines are often referred to as dual channel recorders.
Recording of PPI-type radar signal information on analog rotary-type VTR machines, however, has been fund to have several shortcomings. One shortcoming is that recording noise can cause substantial signal loss; that is, the signal to noise (S/N) ratio may be so low over portions of the bandwidth that, upon reproduction, noise sometimes cannot be distinguished from intelligent signal information.
Another shortcoming of conventional radar recording systems using analog rotary-type VTR machines relates to replay of the information. Recording and replay of signal information by VTR machines of the rotary type inevitably introduces time related errors, usually referred to as time-displacement errors. The time-displacement errors can arise from various mechanical and electrical sources, including tape-motion irregularities, head-motion irregularities, and tape-dimension changes. These factors cause variations in recording and playback speeds which, if not compensated for upon replay, can cause image instabilities. The instabilities are often manifested as picture jitter or skew. In dual channel recorders, inter-channel skew can occur during replay and may prevent the output of one channel from being precisely correlated to the output of the other channel.
In the case of television signal information processed by analog rotary-type VTR machines, time-displacement errors are often compensated for upon replay by time-base corrector (TBC) devices. Modern TBC devices include analog-to-digital converters to digitize the video portion of television signal information, memory elements to store the digitized signals, and digital-to-analog converters to provide analog video output signals. With such TBC devices, the digitized video signal information can be recovered from memory at controlled rates in synchronization with stable, standardized fixed-frequency television timing signals that are normally generated independent of the recorded signals. Such systems can overcome most time-displacement errors by employing the standard fixed-frequency signals.
In contrast, standard fixed-frequency synchronizing signals are usually not available for use in recording signal information provided by radar systems. The radar master trigger signals, which provide synchronization during real-time operation of radar systems, normally do not occur at standard repetition rates. Hence, upon replay of recorded radar signal information from analog rotary-type VTR machines, time displacement errors normally cannot be overcome by conventional TBC devices. Also, in radar systems employing rotating antennae, the rate of antenna rotation may not be constant and such variations may further complicate correction of time-displacement errors. The net result of such complications in conventional analog recording of radar signal information is that recorded target images often will exhibit substantial jitter when replayed for display on a PPI screen.
The preceding discussion has focussed upon analog recordings of radar signal information. The distinguishing feature of analog recordings is that analog input signals are recorded in a time-varying (i.e., analog) manner, usually through modulation techniques. It is well known in the prior art, however, that analog signals can be encoded into digital signals by using binary digits. In the case of television signal information, such digital encoding of analog signals can be accomplished, for example, by the digital TBC devices discussed above. Digital encoding is also widely used with communication signals and with instrumentation and sensor signals. Normally, digital encoding is accompanied by analog-to-digital converter (ADC) devices that take samples of the analog signals at predetermined intervals and, for each sample, quantize the amplitude of the analog signal in terms of a binary code. To obtain accurate representations of analog signals, the sampling rate must be rapid enough to capture critical changes in signal levels. (For a relatively smoothly changing analog signal, the minimum sampling rate normally is several multiples of the signal frequency; thus a one megahertz analog signal, for instance, will ordinarily be sampled at a rate that provides at least two or three million samples each second.) An adequate sampling frequency for replicating radar signal information is normally in excess of about six megahertz. Further to obtain accurate digitally-encoded representations of analog signals, a sufficient number of quantizing levels must be provided to reflect the various amplitudes of the analog signals. The quantizing levels are normally expressed in binary code according to well-known procedures. Although signal replication accuracy is improved by using high bit codes (i.e., numerous quantizing levels), such accuracy is achieved at the expense of transmission time or bandwidth, or both.
To record digitally-encoded signal information on magnetic tape, it is well known to employ devices similar to videotape recorders. Often such devices are referred to as high digital data rate (HDDR) recorders or high-bit-rate (HBR) recorder/reproducers. One example of such recorder/reproducers is the model HBR 3000i recorder manufactured by Ampex Corporation of Redwood City, Calif. A significant advantage of such recorder/reproducer machines is substantial reduction of signal noise problems due to the digital signal processing.
Recording and reproduction of digitally-encoded signal information by machines is normally accomplished by a plurality of fixed (i.e., nonrotary) transducer heads. Typically, HDDR and HBR recorder/reproducers employ recording techniques whereby streams of binary information from several sources, or parallel streams derived from a single source, are recorded simultaneously over a plurality of separate channels. In terms of magnetic recording tape, the channels are recorded on parallel tracks that extend lengthwise (i.e., longitudinally) on the tape. The Ampex HBR 3000i recorder can be configured, for example, to record on either fourteen or twenty-eight parallel longitudinal channels and, in the latter configuration, can record digital information at rates up to about one hundred and fifty megabits per second at tape speeds of about 180 inches per second (ips).
Using the longitudinal tape transport technology of HDDR and HBR recorder/reproducers, it is known to record highly precise synchronizing information simultaneously with the other data being recorded. By employing the synchronizing data during replay, such machines can be operated to substantially minimize time-displacement errors. In the case of Ampex HBR recorder systems, for example, unique synchronization (sync) words are sequentially inserted in parallel data streams that are input to the recorder system; when binary information in a data stream is replaced by a sync word, the replaced information is recorded on a master channel, thus preserving the information. In practice, the process of data removal and replacement is repeated at selected bit intervals such as every 512 bits. During replay of a recording, electronic deskew logic detects the unique sync words in each track, removes corresponding binary information from the master channel and stores the decoded information in registers for simultaneous clocking out by an internal (or external) clock to replicate the original data format. The final result is substantial reduction of time-displacement errors and is a major advantage of such machines.
Conventional HDDR and HBR recorder/reproducers that employ longitudinal (i.e., fixed head) recording techniques have, however, some limitations when recording PPI-type radar signal information. A primary limitation relates to the bandwidth of the information. At normal tape transport speeds, the bandwidth recording and/or reproduction capability of each channel of conventional fixed-head recorder/reproducers are limited to about two or three megahertz because of the characteristics of the fixed transducer heads. To record broader bandwidths, such as required for radar signal information, the conventional approach with such recorder/reproducer machines is to increase tape transport speed. This approach, however, reduces the recording time obtainable from a tape reel and, therefore, may necessitate frequent changing of reels, perhaps as often as several times each hour when radar signal information is recorded.
Bandwidth limitations of fixed transducer heads also limit the input data rates than can be accommodated by HDDR and HBR recorder/reproducers. (The recording and/or reproduction performance of such machines is normally limited to about five to six megabits per second per track at normal tape transport speeds.) To overcome bandwidth limitations of such machines in some instances, it is known that specialized serial-to-parallel digital data converters can be employed. When their use if feasible, such converters accept high data rates (sometimes in excess of 100 megabits per second) and distribute the incoming data streams into a number of lower rate data channels for simultaneous recording in parallel. Such serial-to-parallel converters represent an expense, however, and are not feasible in all circumstances.
At this juncture, it should be emphasized that digital encoding of radar signal information with conventional technology ordinarily requires a sampling rate in excess of about ten megahertz to provide good resolution of the data and to normally require quantization 6 to 8 bits to provide adequate signal-to-noise ratios. The high sampling rates are required with current technology because master trigger signals must be precisely detected and recorded. Thus, if information obtained from broadband sampling of radar signal information is completely recorded, current technology requires HDDR and HBR recorder/reproducers to operate at extraordinarily high tape speeds and to employ several recording channels for recording the master trigger signal information. Occupancy of multiple channels by encoded master trigger information, in turn, limits the number of channels available for other components of the radar signal information, such as echo pulse information and azimuth data. The problem of channel occupancy is compounded if the recorded radar signals include pre-trigger pulses. (Such pulses resemble master trigger pulses but precede them by several milliseconds; generally speaking, pre-trigger pulses are used to control PPI machines by defining the ends of PPI sweeps.) Thus, when conventional HDDR or HBR machines are used to record both master trigger and pre-trigger radar pulses as well as radar echo pulse information using longitudinal recording techniques, recording cycles per machine are substantially reduced, usually to a recording time per reel of only about fifteen minutes or less. Accordingly, notwithstanding the capacity of such recorder/reproducers to record large amounts of binary data in short periods of time, workers in the art have believed such machines were generally not practical or cost-effective for providing extended recordings of complete radar signal information.
Because of the current limitations of HDDR and HBR recorder/reproducers using longitudinal recording techniques to record radar data, signal processing techniques have been developed to selectively sample only certain portions of radar information and then to provide for recording only the sampled information that meets certain predetermined criteria. For example, a sampling system of the type under discussion may operate to sample raw radar data only one per several occurrences of master trigger signals. Such sampling techniques effectively extend recording time per reel because only a fraction of the available information is recorded; however, such sampling techniques are also complex and usually expensive because of required electronic decision-making circuitry. Also, there are situations where sample information is not sufficient because recordings and reproductions of it lack all the intelligence needed for certain purposes.